SUMMARY: In yeast, thiopyronine (TP) acts both as a growth inhibitor (in the dark) and a photodynamic sensitizer (in the light). We have isolated a large number of TP-resistant mutants of and characterized them by genetic and photobiological techniques. Resistance to the growth-inhibitory (dark) effects of TP can arise by mutations in at least four separate genes. Growth-inhibitory and photodynamic effects of TP can be genetically separated, as evidenced by the finding that resistance mutants affected in three of the genes were not altered in their photodynamic response to the dye. In contrast, a mutation in the fourth gene simultaneously conferred high resistance to both the growth-inhibitory and photosensitizing effects of TP, suggesting that a mutation had occurred at a step common to the expression of both effects. Preliminary data suggest that this mutation reduces the ability of the cells either to take up, or bind, TP.


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